Pilbara, located in a far north-western region of Western Australia, is a twofold treasure; not only is it an ancient monument to past geological processes (at the venerable age of over 2,500,000,000 years), it is also the richest source of prehistoric petroglyphs in Australia and perhaps the richest in the world (containing thousands - or even millions - of pecked and engraved figures); the Burrup Peninsula alone is said to contain 700 sites significant to the 30,000 (or greater) year history of the Aboriginal presence in Western Australia. Irrespective of which aspect holds your interest best, it is a vast and fascinating place. So I'll talk about it some more.
The discoverer of the first petroglyphs is unknown. If William Dampier encountered them in 1688, he makes no mention of them in his brief (2,500 word) account of Australia. The first records are those of the crew of the HMS Beagle (Captain John Wickham and Lieutenant John Stokes, who wrote their accounts in 1843 and 1846 respectively). The first systematic study of the latter was conducted by the Western Australian Museum, which sent an expedition to Depuch Island in 1962, shortly followed by the surveying of Bruce Wright (who scanned the area from Roeburne to the Upper Yule River). The most significant analytical work, however, has been done since the 1960s by Robert G. Bednarik and the aforementioned Wright. The ‘Spear Hill Complex,’ discovered in 1982 by Howard McNickle, is the first (and - arguably - most archaeologically profound) of 28 separate sites, some of which are so enormous they could easily be further subdivided.
Although there are some rock paintings, the overwhelming majority of the figures portrayed are petroglyphs created by chipping away the dark patina (weathered surface) of a rock to expose the lighter (nearly golden) rock underneath. This produces a stark contrast and is very striking. Most of the petroglyphs portray stylised human figures (often with exaggerated sexual organs; some are drawn in an ‘X-ray’ style), speared or otherwise wounded animals and birds (generally kangaroos or emus; these comprise the bulk of the figures), flying creatures (of indeterminate identity), entire hunting scenes, tracks (both human and animal) and many purely abstract and/or geometric shapes.
The age of the figures is unknown, but it is most certainly prehistoric (i.e. tens of thousands of years old). In some cases (over 20% of the petroglyphs present, discovered by Bednarik), the dark patination has entirely reformed and micro erosion analysis (a technique which admittedly has a margin of error as high as 50%) suggests that some may be of Pleistocene age, but this is not certain. While most of the paintings were inscribed on vertical rock faces, some occur on fragile horizontal areas which crack if stepped upon - ergo, the study of this antiquity is a delicate process. It is of no assistance that their purpose is essentially unknown; no such information has been passed down to the modern Pilbara Aboriginal people and most surveyors prior to the 1960s observed only individual sites and payed very brief visits.
Many petroglyphs now call the Western Australia Museum (Perth) home.
And then there’s the structure of the rocks themselves - something far more ancient. Pilbara, you see, was one of three large cratons established in Western Australia no later than 2500 million years ago (ma). At this point, only the western third of Australia existed (Pilbara itself comprises a 60,000 square Kilometre area) and it was a harsh volcanic landscape devoid of all but bacterial life. Until 900 ma, these cratons were separated by active, linear mountain chains. The region has endured subduction, rifting and cyclic transpression and transtention… well, as you can see, the technicalities abound. The short of it is that Pilbara provides a great deal of information about past tectonic processes. Most of that information, though, is used to keep Australia at the forefront of mineral exploration and exploitation: mineral resources accounted for over $48 billion in 2001.
Pilbara has a thriving iron ore industry. Much of this comes from banded iron formations (BIFs) - black, grey and red sedimentary rocks, comprised of alternating iron- and silica-rich layers and formed as a result of the oxygen produced by photosynthetic bacteria in the early ocean forming a chemical precipitate. The iron-rich layers are formed of black magnetite (Fe3O4) and dark red-grey haematite (Fe2O3), whereas the silica-rich layers primarily contain fine-grained quartz or chert. The presence of free oxygen in the ocean determined whether the iron (which is of volcanic origin) was precipitated in the oxidised form (Fe3+) in haematite or the reduced state (Fe2+) in magnetite. Although the iron content of many BIFs was originally only around 30% (insufficient to be considered an ore), protracted weathering and erosion have removed non-iron components and increased the iron content to over 60%. Mount Whaleback, Tom Price and Paraburdoo (at 64%) are prime examples.
All that free oxygen had to come from somewhere, and it is well known that Cyanobacteria fossils found in Archæan carbonate rocks in Pilbara (between Marble Bar and Port Hedland) are the cause and, with the oldest of them dated at 3,460 ma, they provide one of the greatest bodies of evidence for early life on Earth. Most, though, are found in Proterozoic rocks (2,500-545 ma) and represent a broad spectrum of ancient life - indeed, most periods of geologic time are represented. The original discovery of these relics was made by Dr. Alex Trendall (former Director of the Geological Survey of Western Australia and now Adjunct Professor of Applied Physics at Curtin Institute of Technology in 1984. The isotope dating work, however was done in 1997 by Prof. Hans Hoffman and Dr. Kath Grey of the University of Montreal. The discovery of ancient stromatolites has inspired NASA to conduct searches for similar evidence of past life on Mars.
Sources:First Dating of Pilbara Petroglyphs, Robert G. Bednarik.
Archaeology: the Definitive Guide, various authors.
Spotlight Earth and Environmental Science, various authors (David Heffernan, John McDougall, Rob Mahon and Kylie Gillies).
http://www.abc.net.au/catalyst/stories/s399058.htm#transcript (Catalyst: ‘Rock Art,’ 25-10-2001).
http://abcasiapacific.com/englishbites/stories/s741422.htm (ABC Asia-Pacific - English Bites: ‘Rock Art,’ 9-12-2002).
http://www.westernaustralia.net/fast_facts/download/pdf/Pilbara_Planner_2003_2.pdf (Pilbara Planner 2003 - a very useful map).
http://www.roberiver.com/resdev/ (ROBE Resource Development: Pilbara Geology).
http://www.geo.uu.nl/Research/StructGeology/beintema.html (Structural Geology and Tectonics: Kike Beintema).
http://www.doir.wa.gov.au/news/31jul01.html (‘Research unveils Pilbara’s geological mysteries’, 31/7/2001).
http://www.agso.gov.au/rural/projects/20011023_26.jsp (‘Geoscience Australia’: North Pilbara Project 1995-2000, 23/10/2001).